1. Field of the Invention
The present invention relates to photolithographic systems used in semiconductor wafer fabrication. More particularly, the present invention relates to systems and methods for reducing optical errors for a pattern projected onto a wafer in a semiconductor fabrication apparatus.
2. Background
Semiconductor wafer fabrication involves a series of processes used to create semiconductor devices and integrated circuits (ICs) in and on a semiconductor wafer surface. Fabrication typically involves the basic operations of layering and patterning, along with others such as doping and heat treatments. Layering is an operation used to add thin layers of material (typically insulator, semi-conductor or conductor) to the surface of the semiconductor wafer. Patterning is an operation that is used to remove or implant dopants into specific portions of the top layer or layers on the wafer surface. Patterning is usually accomplished through the use of photolithography (also known as photomasking) to transfer a semiconductor design to the wafer surface.
Patterning is often used to expose an area to be etched, such as to create a trench for creation of an isolation structure. A ‘reticle’ is a hard copy of a pattern recreated in a thin layer of an opaque material, such as chrome, deposited on a glass or quartz plate. The reticle may be used directly in the patterning process to project a pattern—or image—onto a wafer. Alternatively, the reticle may be used indirectly to produce a photomask. The photomask, or ‘mask’, is also a glass plate with a thin chrome layer on the surface used to pattern or expose an area to be etched. Often, the reticle or mask is covered with many copies of the circuit pattern. The reticle or mask is then used to pattern a whole wafer surface in one pattern transfer.
Photolithographers define optical budget as a combination of resolution, or the minimum line space that can be resolved with a pattern, and depth of focus, the range at which the light patterning projection maintains a predefined accuracy. References to optical budget can also relate to pattern displacement from one layer to another.
Semiconductor device sizes have decreased dramatically over the years. Reaching sub-micron IC feature sizes and patterns has introduced new problems. Currently, photolithographers are looking toward removing the sag of a reticle, or mask, due to gravity as it sits on a reticle stage. Gravitational sag is currently the main obstacle to optical budget management. For six inch by six-inch mask, gravitational sag robs a major portion of the total focus budget. With plans to increase mask size to 9 by 9 inches, gravitational sag is expected to exceed over 60 percent of the optical budget. In addition to gravitational sag, other hardware realities are hindering current patterning techniques.
In view of the above, what is needed are solutions for addressing inaccuracies in photolithography introduced by hardware used in a semiconductor apparatus.